[0001] The present invention relates to a composite organic-coated steel sheet suitable
for use in automobiles and exhibiting improved weldability, electrodeposition coatability,
and press formability as well as excellent corrosion resistance capable of withstanding
so-called "contact rust" corrosion caused by contact with a rusty material.
[0002] Various types of surface-treated steel sheets have been used in a wide variety of
industrial fields including the automobile industry. As the proportion of such steel
sheets used increases, the level of properties required for these steel sheets becomes
higher. In the automotive industry, for example, surface-treated steel sheets are
required to have long-lasting corrosion resistance such as "ten year guarantee against
perforation" as well as good electrodeposition coatability and weldability.
[0003] Among many types of surface-treated steel sheets which have been proposed for use
in automobile bodies, so-called composite organic-coated steel sheets, which comprise
a steel sheet plated with zinc or a zinc alloy having on the plated surface a lower
chromate film and an upper thin organic coating film formed from an organic resin-based
primer, possess significantly improved corrosion resistance due to the anticorrosive
effect of the zinc-based plated coating and the chromate film associated with the
shielding effect of the organic coating as a barrier to shield from the surrounding
environments.
[0004] Although composite organic-coated steel sheets can exhibit their prominent corrosion
resistance under normal corrosive environments, it has been found that they cannot
be prevented from rusting under special corrosive environments. For instance, when
a composite organic-coated steel sheet is assembled with a bare cold-rolled steel
sheet so as to bring these sheets into direct contact with each other, the rust formed
on the bare steel sheet by corrosion adheres to the surface of the composite organic-coated
steel sheet. In such cases, severe corrosion similar to "contact rust" observed on
stainless steels may occur on the composite organic-coated steel sheet. The mechanism
of this phenomenon has not been completely elucidated, but it is thought to result
from adsorption of the rust by the organic coating to accelerate penetration of any
corrosive substance through the organic coating.
[0005] Corrosion of composite organic-coated steel sheets due to "contact rust" has not
received adequate investigation. As a measure for preventing such "contact rust" corrosion
in a composite organic-coated steel sheet, it is considered to be effective to increase
the thickness of the upper organic coating film thereof. However, the thickness of
the organic coating film cannot be increased since such an increase is inevitably
accompanied by a deterioration in electrodeposition coatability and weldability. The
same problem is involved when increasing the crosslink density of the organic coating
film to make the coating more rigid.
[0006] It is known that the corrosion resistance of a composite organic-coated steel sheet
can be further improved by incorporation of fine silica particles such as colloidal
silica in the upper organic coating thereof. See Japanese Patent Application Kokai
No. 2-134238(1990) and U.S. Patent No. 5,147,729. However, these composite organic-coated
steel sheets are not satisfactory with respect to prevention of "contact rust" corrosion.
[0007] It is an object of the present invention to provide a composite organic coated steel
sheet suitable for use in the manufacture of automobile bodies.
[0008] It is another object of the present invention to provide a composite organic-coated
steel sheet having well-balanced properties.
[0009] It is a further object of the present invention to provide a composite organic-coated
steel sheet having a significantly improved corrosion resistance capable of withstanding
"contact rust" corrosion while still having good electrodeposition coatability, weldability,
and press formability.
[0010] The term "good electrodeposition coatability" used herein means that an electrodeposition
coating which is free from gas pinholes and which has good surface appearance can
be formed on the upper organic coating film.
[0011] The present invention provides a composite organic-coated steel sheet comprising
a steel sheet plated with zinc or a zinc alloy and having on the plated surface on
one side or both sides of the steel sheet a lower chromate film and an upper organic
resin coating film, wherein the upper organic coating film has a thickness of 0.1
- 5 µm and is formed from a primer composition which comprises, in an organic solvent,
(1) 100 parts by weight of an epoxy resin having a number-average molecular weight
of from 500 to 10,000, (2) from 10 to 60 parts by weight of an aromatic polyamine
containing from 2% to 30% by weight, based on the aromatic polyamine, of a promoter
selected from the group consisting of phenol compounds and cresol compounds, (3) from
10 to 60 parts by weight of a polyisocyanate, and (4) from 10 to 40 phr of silica
particles of colloidal silica or fumed silica or a mixture of these.
[0012] The term "phr" is an abbreviation of per hundred resin and is used in the context
of the present invention to indicate parts by weight based on 100 parts by weight
of total resin solids which are the sum of the solids of epoxy resin component (1),
the promoter-containing aromatic polyamine component (2), and the polyisocyanate component
(3).
[0013] The present inventors investigated various factors which may have an effect on "contact
rust" corrosion in a composite organic-coated steel sheet and found the following.
(i) The factors in the lower chromate film (coating weight, types and amounts of additives)
have little effect on "contact rust" corrosion.
(ii) Corrosion caused by "contact rust" can be diminished by increasing the thickness
of the upper organic coating film.
(iii) When the resistivity of an organic coating film is determined before and after
a corrosion test using a corrosive solution containing iron rust, the resistivity
significantly decreases after the corrosion test.
[0014] On the basis of these findings, it is thought that the "contact rust" corrosion of
a composite organic-coated steel sheet originates from deposition of iron rust on
the upper organic coating film and the portions of the organic coating film on which
iron rust is deposited are attacked by corrosive factors such as water, oxygen, and
chlorides, resulting in deterioration by expansion or blistering of the organic coating
film and causing corrosion. This indicates that the "contact rust" corrosion is largely
governed by the properties of the upper organic coating film and that in order to
prevent such corrosion, it is important to modify the organic film.
[0015] It has been found that an organic coating film formed from an epoxy resin-based primer
composition comprising (1) an epoxy resin, (2) a promoter-containing aromatic polyamine,
(3) a polyisocyanate, and (4) colloidal silica and/or fumed silica is highly effective
for prevention of "contact rust" corrosion and at the same time exhibits good weldability,
press formability, and electrodeposition coatability.
[0016] The composite organic-coated steel sheet of the present invention is characterized
in that the upper organic resin coating film is formed from such an epoxy resin-based
primer composition with a thickness of 0.1 to 5 µm. The underlying chromate film and
the base zinc-plated steel sheet may be the same as employed in conventional composite
organic-coated steel sheets.
[0017] In the following description, all the parts and percents are by weight unless otherwise
indicated.
Zinc-plated steel sheet:
[0018] The base material in a composite organic-coated steel sheet is usually a zinc-plated
steel sheet, i.e., a steel sheet plated with pure zinc or a zinc-based alloy. The
zinc or zinc-based alloy plated coating may be formed by electroplating, hot dipping
(galvanizing), alloyed galvanizing, or vacuum deposition plating on one or both sides
of a substrate steel sheet.
[0019] The type of the substrate steel sheet is not critical. However, when the composite
organic-coated steel sheet is intended for use in automobiles, it is advantageous
that the substrate steel sheet be of the bake-hardening type, which is designed to
undergo hardening during baking of a finish coating, which is normally applied, in
the manufacture of automobile bodies, after press forming and assembling. A bake-hardening
steel sheet can provide automobile bodies with a significantly enhanced strength by
hardening in the finish coating stage without interfering with the preceding press
forming stage.
[0020] The coating weight of the zinc or zinc alloy plating is not critical, but it is preferably
in the range of from 10 to 60 g/m² for one surface in view of a balance between press
formability and corrosion resistance. The plated coating may be a multilayer coating,
e.g., consisting of a lower zinc or zinc alloy layer and an upper thin layer of a
metal other than zinc (such as iron or nickel) formed by the so-called flash plating.
Lower chromate film:
[0021] The lower chromate film can be formed on the zinc-plated coating by use of a chromating
solution of the coating, conversion, or electrolysis type. Particularly, a chromating
solution of the coating type is preferred since it gives a chromate film having a
particularly improved corrosion resistance.
[0022] A chromating solution of the coating type, when applied to a substrate and baked,
forms a chromate film by reduction of chromic acid and evaporation of water. In order
to accelerate the reduction and film formation by baking at a relatively low temperature,
it is advantageous to employ a two-stage reduction method in which the chromic acid
present in the solution is partly reduced prior to application so as to decrease the
amount of chromic acid to be reduced during baking.
[0023] The coating weight of the chromate film is usually in the range of from 5 to 200
mg/m² and preferably from 30 to 120 mg/m² as Cr metal.
Upper organic coating film:
[0024] The upper organic coating film is formed from an epoxy resin-based primer composition
comprising (1) an epoxy resin, (2) a promoter-containing aromatic polyamine, (3) a
polyisocyanate, (4) colloidal silica and/or fumed silica, and (5) one or more organic
solvents.
(1) Epoxy resin:
[0026] The epoxy resin constitutes a main vehicle of the primer composition used in the
preesnt invention. Epoxy resins known in the art include various classes including
glycidyl ethers, glycidyl esters, glycidyl amines, and linear aliphatic or alicyclic
epoxides. Any of these epoxy resins can be used in the present invention. In addition,
a variety of modified epoxy resins such as acrylate-modified and urethane-modified
epoxy resins are also useful in the invention.
[0027] Preferably, the epoxy resin is of the glycidyl ether type such as bisphenol or novolak
epoxy resins. The bisphenol epoxy resins can be prepared by reacting a bisphenol compound
with an epihalohydrin such as epichlorohydrin. The bisphenol compound useful in this
reaction includes bisphenol A [= 2,2-bis(4-hydroxyphenyl)propane], bisphenol F (=
4,4'dihydroxydiphenylmethane), and bisphenol S (= 4,4'-dihydroxydiphenylsulfone),
and brominated or fluorinated derivatives of these compounds. The novolak epoxy resins
include phenol novolak resins and cresol novolak resins, both of which can be used
in the invention.
[0028] The epoxy resin useful in the invention has a number-average molecular weight in
the range of from 500 to 10,000 and preferably from 1,000 to 5,000. When the number
average molecular weight of the epoxy resin is less than 500, a crosslinking reaction
does not proceed sufficiently to increase the molecular weight of the resin to a desired
level during baking of a wet film formed from the primer composition, thereby deteriorating
the corrosion resistance of the resulting cured organic coating film. An epoxy resin
having a number-average molecular weight of more than 10,000 results in the formation
of an organic coating film having an extremely high hardness, leading to a degradation
in press formability, and it may also result in a degradation in corrosion resistance
due to a loss of crosslink density.
(2) Promoter-containing aromatic polyamine:
[0029] The aromatic polyamine component serves as a curing agent for the epoxy resin. Any
aromatic compound having two or more primary or secondary amine groups can be used
in the invention. Examples of suitable aromatic polyamines include m-phenylenediamine
(MPDA), 4,4'-diaminodiphenylmethane (DDM), m-xylylenediamine (MXDA), 4,4'-diaminodiphenylsulfone
(DDS), and 4-chloro-phenylenediamine (MOCA).
[0030] The aromatic polyamine is used in conjunction with a promoter for the following reason.
The composite organic-coated steel sheet of the present invention is primarily used
in the manufacture of automobile bodies. In such application, a bake-hardening steel
sheet is often used as the steel substrate as described previously.
[0031] When the steel substrate is a bake-hardening steel sheet, the temperature at which
any wet primer film is baked is frequently limited to about 150 °C at highest such
that the bake-hardenability of the substrate steel sheet is not inhibited by premature
hardening during baking of the primer film. However, since the reactivity of an aromatic
polyamine is not so high due to steric hindrance of the amino groups on an aromatic
ring, an extremely prolonged baking time is required to bake a wet film of the primer
composition at such a limited temperature, thereby greatly diminishing the practical
value of the composition.
[0032] In order to avoid such a delay in curing by baking at a relatively low temperature,
a promoter selected from phenol compounds and cresol compounds is used along with
the aromatic polyamine. Useful promoters include such phenol compounds as nonylphenol,
salicylic acid, and m-hydroxybenzoic acid as well as such cresol compounds as cresol.
[0033] The promoter-containing aromatic polyamine component is used in an amount of from
10 to 60 parts and preferably from 20 parts to 45 parts per 100 parts of the epoxy
resin component. When the amount of the aromatic polyamine component (aromatic polyamine
+ promoter) is less than 10 parts, the resulting primer composition forms a cured
organic coating film having a decreased crosslink density, thereby deteriorating the
corrosion resistance. Addition of the aromatic polyamine component in excess of 60
parts also results in a decrease in corrosion resistance since unreacted curing agent
having free amino groups remains in a cured organic coating film in an increased proportion.
[0034] The phenol and/or cresol compound as a promoter is used along with the aromatic polyamine
in an amount of from 2% to 30% and preferably from 5% to 25% based on the weight of
the aromatic polyamine compound. When the amount is less than 2%, the phenol and/or
cresol compound cannot produce a sufficient promoting effect on crosslinking of the
epoxy resin, thereby making it difficult to form a cured organic coating film having
good corrosion resistance. Addition of more than 30% of the promoter to the aromatic
polyamine causes curing of the epoxy resin to proceed excessively, resulting in the
formation of an extremely hard cured organic coating film, which deteriorates not
only the press formability but also the mechanical properties such as flexural and
tensile properties to such a degree that the resulting coated steel sheet no longer
withstands stresses applied in practical applications.
(3) Polyisocyanate:
[0035] The polyisocyanate also functions as a curing agent. Thus, two classes of curing
agents, an aromatic polyamine and a polyisocyanate, are used together in the primer
composition. As a result, silica particles can be present in the primer composition
as a stable dispersion and the composition can form a cured organic coating film which
exhibits improved corrosion resistance and electrodeposition coatability. In the absence
of either one of the curing agents, i.e., either the aromatic polyamine or the polyisocyanate,
the stability of dispersed silica particles in the primer composition as well as corrosion
resistance and electrodeposition coatability of a cured organic coating film are degraded.
[0036] The polyisocyanate component is also present in the primer composition in an amount
of from 10 to 60 parts and preferably from 20 to 45 parts per 100 parts of the epoxy
resin component. When the amount of the polyisocyanate is within this range, improvements
in the stability of dispersed silica particles and in the corrosion resistance and
electrodeposition coatability of a cured organic coating film become significant.
In contrast, these properties are degraded by addition of the polyisocyanate in an
amount of either less than 10 parts or more than 60 parts.
[0037] Examples of suitable polyisocyanates which can be used in the present invention include
aliphatic or alicyclic diisocyanates such as hexamethylene diisocyanate, isophorone
diisocyanate, and hydrogenated diphenylmethane diisocyanate; aromatic diisocyanates
such as tolylene diisocyanate and diphenylmethane-4,4'-diisocyanate; triisocyanates
such as an adduct of 3 moles of one of the above-named diisocyanates to 1 mole of
trimethylolpropane, a trimer of hexamethylene diisocyanate, a trimer of tolylene diisocyanate;
and the like.
[0038] A polyisocyanate curing agent is often used in a blocked form (called blocked isocyanate)
in which the free isocyanate groups have been reacted with a blocking agent. The polyisocyanate
component used in the invention may be either of the blocked type or of the non-blocked
type, although a blocked type polyisocyanate has the advantage of extending the pot
life of the primer composition.
[0039] When the polyisocyanate component is used in a blocked form, it should be blocked
with a blocking agent having a release-initiating temperature below 160 °C such that
the blocking agent can be released by baking of a wet film at a temperature below
160 °C. Examples of such a blocking agent include oxime blocking agents such as methyl
ethyl ketoxime and cyclohexane oxime; phenolic blocking agents such as phenol, p-tert-butylphenol,
and cresol; and ester blocking agents such as ethyl acetoacetate and methyl acetoacetate.
(4) Silica:
[0040] In order to provide a cured organic coating film formed from the primer composition
with improved corrosion resistance and electrodeposition coatability, silica particles
of colloidal silica, fumed silica, or a mixture of these are added in an amount of
from 10 to 40 phr and preferably from 15 to 30 phr such that they are dispersed in
the primer composition.
[0041] Silica in an amount of less than 10 phr is not sufficient to assure that the organic
coating film has a satisfactory level of electrodeposition coatability, while the
presence of silica in excess of 40 phr causes a deterioration in corrosion resistance
and press formability. The silica particles preferably have an average diameter of
primary particles in the range of from 8 to 40 nanometers.
[0042] Among the two types of silica, colloidal silica is preferred because it can be dispersed
in the primer composition into smaller particles and form a dense siloxane network
in a cured organic coating film, thereby contributing to improvement in corrosion
resistance. Although fumed silica can be used, it has a higher tendency toward the
formation of agglomerates in the primer composition so that the average diameter of
fumed silica particles (agglomerated particles) present in the primer composition
is greater than that of colloidal silica particles. A mixture of these two types of
silica may be used.
[0043] The colloidal silica used in the present invention may be in the form of an organosol,
hydrosol, or mixed sol. It is preferred to use a colloidal silica which has been treated
with an alcohol (a monohydric alcohol, a polyhydric alcohol, or a mixture of these)
so as to provide the particles with improved dispersibility.
[0044] The fine silica particles undergo dehydration/condensation and similar reactions
during baking and form a dense network of siloxane bonds, which serves to form a dense
organic coating film and enhance the corrosion resistance of the film.
[0045] The above-described components (1) to (4) are essential in the primer composition
used to form the upper organic coating film in the present invention. If desired,
one or more additional components may be optionally added to the primer composition.
Examples of such optional components include (i) an anticorrosive pigment, (ii) a
color pigment, (iii) a lubricant, and (iv) other additives.
(i) Anticorrosive pigment:
[0046] The composite organic-coated steel sheet of the present invention is well-balanced
with respect to various principal properties desired thereof, including press formability,
corrosion resistance, electrodeposition coatability, and weldability. When the corrosion
resistance is of greater importance, an anticorrosive pigment may be added to the
composition. The amount of the anticorrosive pigment, when added, is generally within
the range of from 1 to 10 phr.
[0047] The anticorrosive pigment is preferably selected from chromate pigments such as strontium
chromate, lead chromate, barium chromate, calcium chromate, zinc chromate, and magnesium
chromate since they can provide improved corrosion resistance, although other anticorrosive
pigments may be employed.
(ii) Color pigment:
[0048] One or more color pigments may be added so as to color the upper organic coating
film. Such a colored film as the uppermost layer provides the coated steel sheet with
an improved aesthetic appearance and makes it easy to distinguish the coated side
when the substrate steel is coated with the primer composition on one side. Since
the upper organic film is very thin, it may be rather difficult to distinguish the
coated side in a moment when the organic film is colorless, thereby causing inconvenience
to workers.
[0049] The color pigment, when added, should be present in the primer composition in a minor
amount sufficient to color the organic coating film. Useful color pigments include
the following:
White pigments - titanium dioxide, zinc oxide;
Black pigments - carbon black;
Red pigments - iron oxide, quinacridone red, insoluble azo pigments, azo lake pigments;
Blue pigments - phthalocyanine blue, Prussian blue, ultramarine blue;
Yellow pigments - iron oxide, benzimidazolone yellow.
(iii) Lubricant:
[0050] In order to assure that the composite organic-coated steel sheet has good press formability,
a lubricant may be present in the primer composition in an amount of from 0.5 to 5
phr and preferably from 1 to 4 phr. The lubricant in an amount of less than 0.5 phr
is not sufficient to improve the press formability. Addition of the lubricant in an
amount in excess of 5 phr deteriorates the electrodeposition coatability.
[0051] Any lubricant can be used in the invention. Examples of suitable lubricants include
polyethylene waxes having a molecular weight of 1,000 - 10,000, carboxylate ester
waxes, polyalkylene glycol waxes, silicone resins, fluorinated resins, and melamine-cyanurate
adducts formed by a reaction of 2,4,6-triamino-1,3,5-triazine (melamine) with 2,4,6-trihydroxy-1,3,5-triazine
(cyanuric acid) and/or its tautomer.
(iv) Other additives:
[0052] In order to improve one or more properties of the upper organic coating film, the
primer composition may further contain one or more other additives conventionally
employed in coating compositions, particularly epoxy resin-based coating compositions.
Examples of such additives include surface modifiers such as silicones and organic
polymers, antisagging agents, dispersants, and thickening agents. Each of these additives
may be added normally in an amount of 0.1 to 5 phr.
[0053] A primer composition used to form the upper organic coating film can be prepared
by admixing the essential components (1) to (4) and optionally one or more additional
components in an organic solvent. For this purpose, any mixing device which has conventionally
been used in the formulation of coating compositions and which includes a dissolver,
ball mill, and sand grinding mill may be employed. The admixing procedure may be performed
in a single step or multiple steps.
[0054] Any organic solvent which can dissolve the base epoxy resin and which can be evaporated
by heating at a temperature below 150 °C can be used in the preparation of the primer
composition. Suitable organic solvents include ketones such as cyclohexanone, isop'2horone,
and methyl isobutyl ketone, and hydrocarbons such as xylene and toluene. Other various
organic solvents including alcohols, ethers, and esters may also be used. The organic
solvent may be a mixed solvent consisting of two or more organic solvents. The amount
of the organic solvent used is adjusted such that the resulting primer composition
has a viscosity suitable for the application technique selected.
[0055] The upper organic coating film can be formed by applying the primer composition to
the surface of the chromate film formed on a zinc-plated steel sheet by means of a
conventional coating device such as a roll coater, a spray coating machine, or a curtain
flow coater.
[0056] The thickness of the organic coating film is in the range of from 0.1 to 5 µm and
preferably from 0.6 to 1.6 µm on a dry basis. A coating film having a thickness of
less than 0.1 µm is too thin to improve the corrosion resistance to a satisfactory
degree, while a coating film thicker than 5 µm deteriorates the weldability. However,
in those applications where no welding is performed in assembling, the upper organic
coating film may have a thickness in excess of 5 µm.
[0057] The wet film of the primer composition is baked at a temperature in the range of
80 - 300 °C for a sufficient time to cure the coated film, thereby providing a composite
organic-coated steel sheet according to the present invention. When the steel sheet
is of the bake-hardening type, it is preferred to bake the wet film at a temperature
below 200 °C and more preferably below 150 °C as discussed previously. Most preferably,
the baking temperature is between 130 and 150 °C.
[0058] The upper organic coating film formed in this manner is improved in respect to both
corrosion resistance and electrodeposition coatability. Such improved properties cannot
be obtained when the composition is free from either the aromatic polyamine or the
polyisocyanate.
[0059] Although the reason for the improved properties is not completely clear, it is estimated
that the uniform dispersion of the silica particles in the primer composition is maintained
in the presence of the two classes of curing agents throughout the entire length of
time until it is applied and cured, thereby forming a cured organic coating film having
a dense network of siloxane bonds formed from the silica particles. Such a dense coating
has an increased barrier effect on penetration of external corrosive substances through
the coating film, leading to improved corrosion resistance. Furthermore, the dense
coating film minimizes unevenness of the resistivity of the coating film and prevents
passage of a local abnormal current during electrodeposition coating, thereby forming
a uniform electrodeposited coating having a good surface appearance.
[0060] The following examples are presented to further illustrate the present invention.
These examples are to be considered in all respects as illustrative and not restrictive.
EXAMPLES
(1) Substrate steel sheet:
[0061] The substrate steel sheet used in this example was a zinc-plated steel sheet which
had a Zn-13% Ni alloy electroplated coating with a coating weight of 30 g/m² on each
surface of a 0.7 mm-thick cold-rolled steel sheet. Before use, the zinc-electroplated
steel sheet was degreased with a commercially-available alkaline degreasing agent.
(2) Chromate treatment:
[0062] A chromate film was formed on the zinc alloy plating on one side of the degreased
plated steel sheet by applying a commercially-available chromating solution of the
coating type (Surfchrome 92; Nippon Paint) with a bar coater and baking at an HMT
(highest metal temperature) of 80 °C for 10 seconds to give a chromate film having
a weight of 60 mg/m² as Cr metal.
(3) Preparation of primer compositions:
[0063] Primer compositions having the compositions shown in Table 1 were prepared from an
epoxy resin, an aromatic polyamine, a promoter, a polyisocyanate, silica, and optionally
a lubricant and/or a pigment selected from the list given below.
[0064] The procedure for preparing the primer composition of Example 1 was as follows:
(i) The following components were weighed into a glass bottle and thoroughly stirred
with glass beads on a paint shaker to give a silica-dispersed resin solution A which
was free from a curing agent.
Epikote 1004 (epoxy resin) |
25.0 g |
Hakusol S-200 (colloidal silica) |
30.8 g |
Ceridust 3620 (polyethylene wax lubricant) |
0.5 g |
MC-600 (melamine-cyanurate lubricant) |
0.5 g |
Cyclohexanone (organic solvent) |
15.0 g |
Total |


|
(ii) To the glass bottle containing the silica-dispersed resin solution A, the following
components were further added and the resulting mixture was further stirred. Thereafter,
the mixture was filtered through a 200 mesh screen to give a curing agent-containing
epoxy resin-based primer composition.
m-Phenylenediamine (diamine) containing 1.1 g of nonylphenol (promoter) |
8.6 g |
Coronate HX (diisocyanate) |
8.8 g |
Cyclohexanone (organic solvent) |
10.0 g |
|
Total


|
[0065] When the primer composition contained a pigment (anticorrosive pigment or color pigment),
the pigment was added and dispersed along with silica in the silica-dispersed resin
solution A.
(4) Components used in the preparation of primer compositions:
[0066] The following components were used in the primer compositions prepared in the examples
and comparative examples. In the list given below, Mn is the number-average molecular
weight and NV is the nonvolatile content (wt%).
(i) Epoxy resins:
(a) Bisphenol A epoxy resins
(A) Epikote 828 (Shell Chemical; Mn=400, NV=100%)
(B) Epikote 1001 (Shell Chemical; Mn=1000, NV=100%)
(C) Epikote 1004 (Shell Chemical; Mn=2000, NV=100%)
(D) Epikote 1007 (Shell Chemical; Mn=4000, NV=100%)
(b) Bisphenol F epoxy resin
(E) Epotote YDF 2004 (Toto Kasei; Mn=1900, NV=100%)
(c) Phenol novolak epoxy resins
(F) Epiclon N740 (Dai-Nippon Ink and Chemical; Mn=540, NV=100%)
(G) Epiclon N775 (Dai-Nippon Ink and Chemical; Mn=1000, NV=100%)
(d) Cresol novolak epoxy resins
(H) Epiclon N673 (Dai-Nippon Ink and Chemical; Mn=900, NV=100%)
(I) Epotote YDCN 701 (Toto Kasei; Mn=1700, NV=100%)
(J) Epotote YDCN 704 (Toto Kasei; Mn=3050, NV=100%)
(e) Phenoxy epoxy resin
(K) Phenotote YP 50 (Toto Kasei; Mn=11800, NV=100%)
Among the above epoxy resins, Epoxy Resins (A) and (K) are comparative components
since they have a molecular weight outside the range defined herein (500 - 10,000).
(ii) Aromatic polyamines:
(A) m-Phenylenediamine (NV=100%)
(B) 4,4'-Diaminodiphenylmethane (NV=100%)
(iii) Promoters:
(A) Nonylphenol (NV=100%)
(B) Salicylic acid (NV=100%)
(iv) Polyisocyanates:
(a) Hexamethylene diisocyanate
(A) Coronate HX (Nippon Polyurethane; NV=100%)
(b) Isophorone diisocyanate
(B) Desmodur Z-4370 (Sumitomo Bayer Urethane; NV=70%)
(c) Isophorone diisocyanate (blocked with methyl ethyl ketoxime)
(C) Desmodur BL-4165 (Sumitomo Bayer Urethane;
NV=65%, release-initiating temp.=140 - 160°C)
(v) Silica:
(a) Colloidal silica
(A) Hakusol S-200 (Tohaku Naruko; NV=35%, average diameter of primary particles=20
nm)
(b) Fumed silica
(B) Aerosil 300 (Nippon Aerosil; NV=100%, average diameter of primary particles=8
nm)
(vi) Lubricants:
(a) Polyethylene wax
(A) Ceridust 3620 (Hoechst Japan)
(b) Melamine-cyanurate adduct
(B) MC-600 (Nissan Kagaku)
(vii) Pigments:
(a) Color pigment
(A) Indian red (red iron oxide pigment)
(b) Anticorrosive pigment
(B) Barium chromate
(5) Formation of upper organic coating film:
[0067] After the epoxy resin-based primer composition prepared in (3) above was diluted,
if necessary, with cyclohexanone to adjust the viscosity to an appropriate level for
coating, it was applied on the chromate film formed in (2) above by means of a bar
coater so as to give a coating thickness of 1 µm on a dry basis. The wet film was
cured by baking at an HMT of 150 °C for 20 seconds to give a cured organic coating
film as the upper layer of a composite organic-coated steel sheet.
(6) Testing procedures:
[0068] The corrosion resistance, electrodeposition coatability, and weldability of the composite
organic-coated steel sheet were evaluated by the testing procedures described below.
The test results are summarized in Table 2.
(i) Corrosion resistance:
(a) Resistance to "contact rust" corrosion:
[0069] A flat intact test specimen of each composite organic-coated steel sheet was subjected
to 1000 cycles of an accelerated corrosion test in which each cycle consisted of immersion
in a 5% NaCl solution at 40 °C for 7.5 minutes, exposure to a moist atmosphere (RH=95%)
at 40 °C for 15 minutes, and drying at 60 °C for 7.5 minutes. The NaCl solution used
in this test contained a large amount of iron rust which had been formed by introducing
an uncoated steel sheet into the solution and allowing it to corrode therein. After
the corrosion test, the organic-coated surface of the test specimen was visually inspected
with respect to the formation of rust. The corrosion resistance was evaluated on the
basis of the percent area covered by rust on the surface of the test specimen.
(b) Corrosion resistance after press forming:
[0070] Press forming was performed on a test specimen of each composite organic-coated steel
sheet by subjecting it to cylindrical deep drawing with a diameter of 50 mm and a
depth of 50 mm. The shoulder of the die which was used was ground with a #120 Emery
paper before each press forming test. The press-formed test specimen was subjected
to 200 cycles of an accelerated corrosion test in which each cycle consisted of spraying
with a 5% NaCl solution at 35 °C for 4 hours, drying at 60 °C for 2 hours, and exposure
to a moist atmosphere (RH=95%) at 50 °C for 2 hours. Thereafter, the corrosion resistance
was evaluated on the basis of the percent area covered by rust in the deformed portion
of the test specimen.
(ii) Electrodeposition coatability
[0071] Electrodeposition coating was performed on a test specimen of each composite organic-coated
steel sheet using a commercially-available paint for electrodeposition (Power-Top
U-50; Nippon Paint) by passing an electric current under such conditions that a 20
µm-thick coating was electrodeposited on a chromated steel sheet and then baking for
25 minutes at 165 °C to form an electrodeposited coating on the surface of the organic
coating film of the test specimen. The electro-deposition coatability was evaluated
by visual observation of the appearance of the electrodeposited coating as follows:
- ⓞ
- : Smooth and excellent surface appearance,
- ⃝
- : Slightly roughened surface but good appearance,
- △
- : Significantly roughened surface or craters formed,
- X
- : Many craters formed or incapable of electrodeposition coating.
(iii) Weldability
[0072] Two test specimens of each composite organic-coated steel sheet were laid one over
the other such that the organic-coated surface of one specimen faced the uncoated
surface of the other specimen and were subjected to spot welding using an AC single
spot welder with electrodes having a tip diameter of 6 mm under the following conditions:
a welding current of 10,000 A, a weld time of 12 cycles, and a welding force of 200
Kg. The weldability was evaluated in the following two ways.
A. Stability of current passage:
[0073] After 1000 spots were welded, 100 spots were sampled at random and the number of
spots having indentations which were not stable or normal was counted to evaluate
the stability of current passage.
B. Diameter of electrodes after continuous spot welding:
[0074] After 1000 spots were welded, the tip diameters of the electrodes were measured using
a pressure-sensitive paper and evaluated as follows:
- ⃝
- : Tip diameter < 7.0 mm,
- △
- : Tip diameter = 7.0 - 8.0 mm,
- X
- : Tip diameter > 8.0 mm.

[0075] As can be seen from Table 2, the composite organic-coated steel sheet according to
the present invention were improved with respect to all the properties tested. Namely,
they had excellent corrosion resistance capable of withstanding "contact rust" and
press forming, and at the same time they exhibited satisfactory electrodeposition
coatability, press formability, and weldability. Therefore, these steel sheets are
particularly suitable for use in automobile bodies and enable the automobile bodies
to have an extended durability. In contrast, when the molecular weight of the base
epoxy resin or the proportion of one or more components was outside the range defined
herein, at least one of the tested properties was degraded.
[0076] It will be appreciated by those skilled in the art that numerous variations and modifications
may be made to the invention described above with respect to specific embodiments
without departing from the spirit or scope of the invention as broadly described.
1. A composite organic-coated steel sheet comprising a steel sheet plated with zinc or
a zinc alloy and having on the plated surface on one or both sides of the steel sheet
a lower chromate film and an upper organic resin coating film, wherein the upper organic
coating film has a thickness of 0.1 - 5 µm and is formed from a primer composition
which comprises, in an organic solvent,
(1) 100 parts by weight of an epoxy resin having a number-average molecular weight
of from 500 to 10,000,
(2) from 10 to 60 parts by weight of an aromatic polyamine containing from 2% to 30%
by weight, based on the aromatic polyamine, of a promoter selected from the group
consisting of phenol compounds and cresol compounds,
(3) from 10 to 60 parts by weight of a polyisocyanate, and
(4) from 10 to 40 phr of silica particles of colloidal silica or fumed silica or a
mixture of these, phr being based on the sum of components (1), (2), and (3).
2. The composite organic-coated steel sheet of Claim 1, wherein the zinc or zinc alloy
plating has a coating weight in the range of 10 - 60 g/m² for one surface.
3. The composite organic-coated steel sheet of Claim 1 or 2, wherein the chromate film
has a coating weight in the range of 5 - 200 mg/m² as Cr metal.
4. The composite organic-coated steel sheet of any one of Claims 1 to 3, wherein the
upper organic coating film has a thickness in the range of 0.6 - 1.6 µm.
5. The composite organic-coated steel sheet of any one of Claims 1 to 4, wherein the
epoxy resin is selected from the group consisting of glycidyl ethers, glycidyl esters,
glycidylamines, linear aliphatic or alicyclic epoxides, and modified epoxy resins.
6. The composite organic-coated steel sheet of any one of Claims 1 to 5, wherein the
isocyanate groups of the polyisocyanate are blocked with a blocking agent having a
release-initiating temperature below 160 °C.
7. The composite organic-coated steel sheet of any one of Claims 1 to 6, wherein the
organic solvent is selected from the group consisting of ketones, hydrocarbons, alcohols,
ethers, and esters.
8. The composite organic-coated steel sheet of any one of Claims 1 to 7, wherein the
primer composition further comprises one or more additives selected from the group
consisting of anticorrosive pigments, color pigments, lubricants, surface modifiers,
antisagging agents, dispersants, and thickening agents.
9. The composite organic-coated steel sheet of Claim 8, wherein the primer composition
comprises a pigment selected from an anticorrosive pigment in an amount of 1 to 10
phr, a color pigment sufficient to color the upper organic coating film, and a mixture
of these.
10. The composite organic-coated steel sheet of Claim 8 or 9, wherein the primer composition
comprises a lubricant in an amount of 0.5 - 5 phr.